I'll use a defender for low defence low attack monsters like a yak or armoured zombie because of the str bonus, and attack. Otherwise barrows, ect i'd use a shield.
It depends on your style of play and what you're doing. 90% of the population will say Str > Defence, I happen to be in the 9% which say Defence > Str (The other 2% doesn't care either way).
Defenders are good for giving extra Strength bonus. Each strength bonus equates to 1-2 extra damage (meaing you'd hit 201 instead of 200 or 602 instead of 600). Rune def = +10-20, D Def = +12-24 dmg.
